One way is to write a minimal class that consumes the role and then write tests against the class.

Also, you might want to test your consuming classes anyway, because, you need to make sure they are consumed in the way you expect, and you might have different return values or arguments that need to be tested. For example, I have a project where two different classes consume a role. One class returns 'undef' on failure and another has to, because of what the class actually does, return '' on failure. These situations are similar but different and need to be tested appropriately.